(1) Field of the Invention
The present invention relates to a bolt shear force sensor for use in design testing as well as in-service monitoring in order to provide a structural assessment of a bolt or fastener while attached in an operating system.
(2) Description of the Prior Art
The design of submarine and ship systems required to survive the effects of mechanical shock must consider not only the system and foundation to which the system is attached but also the mechanical fasteners connecting them. While it is well known that structures typically fail at mechanical joints and other discontinuities; there are significant difficulties in assessing and predicting localized distribution of forces within the mechanical fasteners used at the joints.
These difficulties increase when loading forces are dynamic as in the case of shock, blast, vibration and seismic events. As such, making a determination of proper sizing, spacing, pre-tensioning and quality of fasteners becomes increasingly critical. Therefore, achieving the required level of structural integrity in mechanically fastened joints has further implications on the design methods used and the need for experimental testing.
In commercial applications, qualification by analysis is becoming more prevalent; especially when providing design guidance and establishing performance criteria. Since the need remains to understand how mechanically-fastened joints behave; a need also exists to identify individual fastener behavior from structural tests and to improve the knowledge base and design methods for joints in load-carrying structures.
Multiple devices and methods for measuring axial forces in fasteners are described in the art. However, these devices and methods do not adequately measure fastener shear forces in which those forces transversely align with a longitudinal axis of a bolt. In a properly designed joint, axial and shear forces must be considered because of their cumulative effect on fastener stress.
While handbook calculations and numerical methods are typically employed for estimation purposes; both types of methods require kinematic and constitutive (stiffness) assumptions in regard to the transfer of external loads and the resulting distribution of internal reaction forces among participating joint components. In mechanically fastened joints subjected to shearing loads; factors (such as elasticity of the joined members, hole tolerances, interfacial friction, creep, thermal expansion, etc) affect the ability of the joint to resist shearing motions among joined members.
While fastener shear forces directly create shear stresses; the forces also create a secondary effect that generates fastener bending stresses. These bending stresses, which are difficult to quantify because the bending moment arm in each fastener is often unknown, superimpose with pretension stresses. As such, it is apparent that for joints subject to shearing loads; fastener tensions stresses comprise the direct axial tension (the sum of initial pretension and externally applied axial joint loads) and the bending stress due to shearing forces.
In the known art, Slack (U.S. Pat. No. 4,870,866) describes an ultrasonic method for measuring contact pressures on mating interfaces of solids undergoing compression. In the reference, an interfacial region contains an entrapped film of liquid. By monitoring the change in acoustic pulses; the contact pressure can be determined. However, to determine the shear force, additional processing would require an integration of the contact pressure over the contact area. Hence, to obtain the shear force; the contact area must also be monitored. This method would be difficult to employ with existing in-service monitoring applications.
Hay (U.S. Pat. No. 5,945,665) describes a fiber optic strain gage-based transducer which measures only the axial force (including pretension) in the bolt (See FIG. 1). In the figure, a Bragg grating sensor 1 is inserted and held by epoxy 2 in a mechanical fastener 10. Optical fiber 3 connects the grating sensor 1 to an external connector 4. Other devices use washers instrumented with wire (foil) strain gages in which the gages only measure the axial forces in bolts.
In other commercially-available measurement devices, pressure sensitive films are mounted between the fasteners. The bearing surface of the mating holes for the fasteners can be used to measure shear forces from patterns of contact pressures. These films contain microcapsules that release color at prescribed pressure ranges. A scanning device is then used to measure the variations in color intensity of the contact pressure pattern. However, the films do not allow continuous, real-time monitoring of forces. Disassembly of the joint is required to quantify the magnitudes of the shear forces.
Also in the prior art, Shah (U.S. Pat. No. 8,433,160) includes optical sensing elements positioned through a continuous aperture collinearly with a longitudinal axis of a fastener in order to sense strain and temperature. The sensing elements sense localized fastener deformations that occur along the longitudinal axis. The fiber optic sensing elements can be used to capture axial, transverse and torsional strains as measured from the localized deformations of the drilled hole region of the fastener. However, the Shah reference cannot distinguish directly between transverse compression strains due to Poisson's effect and bolt bearing strains.
Using FIG. 2, consider an element of material 11 of a fastener 12 loaded in pure axial tension. By Poisson's effect, a compressive strain would develop along an axis orthogonally transverse to the longitudinal axis of the fastener (See FIG. 3). The Shah reference senses this transverse strain which is not a result of any bolt bearing induced strains.
Using FIG. 4, now consider the combined presence of Poisson's strains from bolt tension and transverse strains of a bolt bearing. For example in the figure, the fastener 12 with the identified element 11 is loaded in double shear by forces “P” and “P/2” with the fastener also under axial tensile force as indicated by direction arrows A and B. In FIG. 5, the identified element 11 is under compressive and axial strain.
These transverse strains are sensed rather than the individual transverse strain components. With the use of signal processing, the Poisson's effect transverse strains (i.e., the transverse compressive strains resulting from axial tensile loads in the fastener) must be separated from the transverse bolt bearing strains. This is an indirect approach.
Based on the existing art, a need still exists for a bolt shear force sensor for design validation testing as well as in-service monitoring for structural health assessment. The sensor should be available for use in both static and dynamic applications.